EFFECT OF POLY(VINYLPYRROLIDONE) ON TRANSFORMATION OF THE DISPERSED PHASE AND GELATION IN A LYOPHOBIC COLLOID SYSTEM

Transformation of the dispersed phase and subsequent gelation in an aqueous colloidal dispersion of an organic compound upon aging of the dispersion are analyzed using cryotransfer transmission electron microscopy, video-enhanced optical microscopy, and FT-Raman spectroscopy. After a few hours at room temperature, the original particles of the dispersion (prepared using only an anionic surfactant as the wetting agent) gradually disappear and are replaced by a new set of particles that are strongly anisodiametric. Entanglement of these strongly anisodiametric fibrillar units results in the formation of a network and gelation. FT-Raman spectroscopy suggests that there is structurally greater ordering of the organic compound in the fibrillar units compared to the original particles which appear to be amorphous or liquidlike. We postulate that the original particles represent a metastable state. It is possible that transformation of the dispersed phase (i.e., disappearance of the metastable form and appearance of the thermodynamically stable form) occurs via a dissolution-growth process similar to that associated with Ostwald ripening. The presence of the nonionic polymer poly-(vinylpyrrolidone) in the formulation in conjunction with the anionic surfactant significantly increases the lifetime of the metastable state. In this case the dispersion remains as a sol for a relatively long period of time (over 20 days at room temperature).